In a computed tomography (CT) system, an X-ray image is generated from the body of a patient to be examined from different polar angle directions relative to his body axis in each case and subsequently, from the individual X-ray images which each represent the absorption of the X-rays by the body tissue for the respective angle directions, a three-dimensional volumetric model of the body tissue is reconstructed. The spatial resolution of this volumetric model depends, firstly, on the spatial resolution of the individual X-ray images, that is, on the resolving power of the X-ray detector and, secondly, on the absorption contrast, i.e. in how much detail the varying absorption of the X-ray radiation that is irradiated from the relevant angle onto the body tissue can be represented in a single X-ray image. In order herein always to be able to keep the radiation dose for the patient under examination to a medically acceptable level, X-ray detection which is also still able, for a relatively low radiation dose in a single X-ray image, to represent the different absorption by different tissue layers with sufficient contrast is advantageous.
Both for the greatest possible spatial resolution as well as for the most contrast-rich representation possible of different absorption levels with simultaneously moderate X-ray radiation, quantum-counting X-ray detectors have proved to be advantageous. In a quantum-counting X-ray detector, an incident X-ray photon initially generates a free electron in a semiconductor, for example cadmium telluride, by ionization of one of the lattice atoms which, as a consequence of its remaining kinetic residual energy, in turn ionizes further lattice atoms, so that an electron-hole-pair cloud forms in the semiconductor. The electrons or holes are now each collected by pixelated anodes or cathodes to each of which a bias voltage is applied. On arrival of an electron cloud at an anode pixel, therefore, a current pulse and from this, via a pre-processing, usually a voltage pulse is generated from which the relevant X-ray photon which was responsible for the generation of the electron cloud in the region of the anode pixel can be inferred.
Due to the effects of mirror charges, however, an X-ray photon especially in the boundary region of two detector pixels can lead, through induction to a current pulse or a voltage pulse in the adjacent pixel, i.e. by way of its charge cloud, the X-ray photon generates a corresponding voltage pulse in the detector pixel at the location of its arrival and, in the region of the adjacent pixel, as a result of the charges induced there, a further usually weaker, voltage pulse. This can lead, as a consequence of the counting events which are evoked only though induction from an adjacent region and not through a separate X-ray photon itself, to a worsening of the image contrast. Particularly in the case of a high spatial resolution which is actually desirable for computed tomography, this can become a problem especially due to the edge region being enlarged relative to the overall area of a pixel, in which edge region such induced charges can occur.
Quantum-counting detectors allow a particular minimum energy to be set below which X-ray photons cannot be detected at all. In order to filter the induction-based counting events, that is voltage pulses, which have actually been created by an X-ray photon in an adjacent pixel, the minimum energy can be increased far enough that the induction-based voltage pulses which are weaker than the voltage pulses of the “correct” counting events are no longer registered. This, however, has the disadvantage that also per se “correct” voltage pulses with correspondingly low energy of the causative X-ray photon are no longer detected, which again worsens the image contrast.